1. Field of the Invention
The present invention relates to glass sheet tempering apparatus and particularly relates to the tempering of large glass sheets, especially those that are shaped prior to being tempered. When glass sheets are tempered, each glass sheet in turn is heated above its annealing range and then rapidly cooled to set the surfaces of the glass sheet while the center is still hot. This action results in the sheet having its surfaces stressed in compression while the intermediate portion is stressed in tension.
The stress pattern imparted to temper glass results in a much stronger sheet than untempered glass, because the glass surfaces, by virtue of being stressed in compression, are much more able to withstand external forces than untempered glass sheets which are not provided with such large compression stresses in the surface area. Moreover, when the outer surface of the glass is penetrated, tempered glass breaks up into small, relatively harmless, smoothly surfaced particles. In contrast, annealed glass fractures more readily, and when fractured, breaks into relatively dangerous, large, jagged fragments.
The uniformity of the size of the shattered particles indicates the uniformity of temper of the glass. The smaller, smoother particles of shattered tempered glass are much safer than the jagged fragments of untempered glass.
More specifically, in a typical tempering operation, a glass sheet is heated nearly to its softening point and then quickly chilled by uniformly exposed the opposite surfaces of the heated glass sheet to streams of a tempering fluid, such as air, arranged to cool both surfaces uniformly and simultaneously. The fluid is disposed through two opposed, spaced plenum chambers, each provided with a set of nozzles. Each set of nozzles faces a different major surface of the glass sheet.
The prior art considered it a prerequisite to uniform tempering to have an even distribution of the cooling air over the glass surfaces. This is usually accomplished by blasting air through a plurality of identical, uniformly spaced, elongated nozzles extending through apertures in apertured walls of the plenum chambers. The nozzles are either reciprocated transversely of their length through an amplitude sufficient to insure that each increment of the glass sheet area is swept by at least one of the reciprocating nozzles. This distance between the nozzle orifices and the adjacent sheet surfaces have been kept as uniform as possible in order to strive for the goal of uniform tempering of the glass sheet.
It is necessary to impart relative movement between the nozzles moving in unison relative to the glass sheet to avoid nonuniform cooling of the glass. When the nozzles are not moved relative to the major glass surfaces or vice versa, the tempering medium blasts are directed against fixed locations on the glass. Fixed air blasts cool the fixed locations opposite the blasts rapidly while other locations adjacent to the fixed locations are not cooled as rapidly. Without such relative movement, patterns of iridescence form on the surface of the tempered glass. These patterns of iridescence are very annoying when viewed in reflection or in polarized light.
By providing relative movement of the nozzles relative to the major surfaces of the glass sheet, and by applying the stream of air or other tempering medium through the nozzles by pressure from a common source, prior art tempering apparatus provided substantially uniform tempering for flat glass and gently curved glass of relatively small and intermediate sizes. However, as the size and/or shape of automobile backlights and sidelights became larger and more complicated, it has become more and more difficult to temper glass sheets adequately. It has become necessary to supply air or other tempering medium at a greater rate of flow per unit area for larger sizes than for smaller sizes in order to assure that the glass is adequately tempered.
The glass sheet tempering art has developed many techniques for imparting relative motion between the nozzles that face the opposite surfaces of the glass sheet and the major surfaces of said sheet. Some of these involve linear reciprocation of the nozzles in unison. Others involve linear reciprocating movement of glass sheets past a pair of arrays of fixed opposing nozzles. Others involve applying elliptical or circular orbital movement of nozzles relative to a glass sheet supported at a fixed position.
The prior art recognized that one of the problems of inadequate tempering of large glass sheets and or those having complicated curvatures resulted from the inability of the air blasted against the central portion of the glass sheet to escape from between the central portion of the glass sheet and the apertured walls of the plenum chambers so as to enable fresh cool tempering medium to replace the spent tempering medium that impinged on the glass. The prior art recognized the correlation of the long escape path from the center to the edge of the glass sheet with inadequate center portion temper. According to one proposal to solve this problem, the wall of each plenum chamber facing the central portion of a glass sheet undergoing quenching has a greater proportion per unit area apertured than the remainder of the wall facing the portion of the glass sheet surrounding the central portion. Such a construction causes a slight pressure gradient in the tempering medium from the central region to the outermost regions of the space within which the glass sheet is supported between the plenum chambers for tempering. This slight pressure gradient results in a continuous outward flow from the central portion of the glass to its margin and helps remove air from the vicinity of the glass sheet surface after the relatively cool air supplied through the apertured wall of the plenum chamber has engaged the heated glass surface to chill the latter and has in turn been heated by said engagement.
Providing larger openings in the apertured walls of the plenum chamber in the center portion than in the portions beyond the central portion requires an increase in power to operate compressors or fans that supply cool air to the plenum chambers for delivery through the various nozzles. In view of the increasing cost of energy in recent years, it would be desirable to develop an alternate technique that does not involve the use of so much energy to develop a desired degree of temper.
2. Description of Patents of Interest
U.S. Pat. No. 3,186,815 to Jochim discloses a glass tempering apparatus designed to temper different portions of the glass to different degrees of temper by providing a separate set of nozzles moveable relative to the remaining tempering nozzles in a direction parallel to the thickness of a glass sheet being tempered. The purpose of this invention is to provide different portions of the tempered glass sheet with different properties that are associated with different degrees of temper.
U.S. Pat. No. 3,294,519 to Fickes discloses glass sheet tempering apparatus in which air under pressure is supplied to a pair of opposed plenum chambers and imparted through nozzles having a larger proportion of tempering medium-imparting area per unit cross section area in the central portion compared to that of the portions exterior of the central portion. The purpose of this patent is to increase the flow rate of tempering medium against the central portion of the glass sheet undergoing tempering so as to cause a pressure gradient in the tempering medium parallel to the major surfaces of the glass sheet from the central region to the marginal portion of the glass sheet.
The pressure gradient so established permits tempering medium to escape more readily from the central portion of the glass sheet after it chills the glass surfaces, permitting the application of additional tempering medium, particularly in the central portion of the glass sheet. Establishing the pressure gradient in this manner provides additional power to provide additional flow of cold tempering medium in the central region of the plenum chamber which faces the central region of the glass sheet. The need for additional power consumption to insure adequate temper in the central portion of large glass sheets leaves something to be desired and it would be desirable for the glass sheet tempering art to develop a way of tempering glass sheets at minimum power consumption while permitting the tempering medium applied to the opposite major surfaces of the glass sheet to escape readily throughout the entire extent including its central portion between the apertured walls of the plenum chamber and the major surfaces of the glass sheet.